log: log-engine slight redesign

The messages are filtered by their type

    LOG_MSG     - plain messages, they escape any (!) log level
                  filtration and go to stdout
    LOG_ERROR   - error messages
    LOG_WARN    - warning messages
    LOG_INFO    - informative messages
    LOG_DEBUG   - debug messages

By default the LOG_WARN log level is used, thus LOG_INFO
and LOG_DEBUG messages will not appear in output stream.

pr_panic helper was replaced with pr_err, pr_warning
shorthanded to pr_warn and old printk if rather pr_msg
now.

Because we share messages between "show" and "dump" actions,
before the "show" action proceed we need to tune up
log level and set it to LOG_INFO.

Also note that printing of VMA and siginfo now
became LOG_INFO messages, it was not that correct
to print them regardless the log level.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parallels.com>